1. Field
This embodiment relates generally to nuclear reactor internals and more particularly, to neutron source rod assemblies that are involved during the startup and shutdown phases of a reactor core.
2. Related Art
The neutron source rod assemblies provide a base neutron level to ensure that X-core neutron detectors are operational and respond to source level neutrons during the period of low neutron activity such as for the initial core loading during the reactor startup. This is also the case during reactor shutdown for reloading and maintenance. The neutron source rod assemblies also permit detection of changes in the core multiplication factor during core loading and approach to criticality. The neutron sources used have included those referred to as “primary sources” and “secondary sources.” Primary sources are those which are made of neutron emitting isotopes in the form in which they are initially placed in the reactor core during initial core loading. Secondary sources are those which are made of initially non-neutron emitting materials, which become neutron emitters during its residence in the first cycle of operation in the reactor core. The secondary sources are the neutron sources that are used during the subsequent shutdowns until the end of life of the secondary sources. Typical of the materials utilized for neutron sources are those including combinations of Polonium and Beryllium, Plutonium and Beryllium, Antimony and Beryllium, Americium-Beryllium and Curium, and sources including Californium. These sources can be relatively expensive. Because secondary sources are irradiated in-core, they are typically less expensive to ship than the primary sources. The secondary sources are easier to transport and handle as they are initially non-radiating.
The neutron sources are utilized during core startup and shutdown to ensure the operability of monitoring and detection apparatus, such as neutron detectors aligned with the reactor core and positioned outside of the reactor vessel. This is in accordance with governmental regulations applicable to the nuclear industry which dictate that means must be provided for monitoring or otherwise measuring and maintaining control of the fission process under all operating conditions, including shutdown.
Accordingly, primary neutron sources for commercial reactors have been positioned within the nuclear core, and remain within the core during at least one entire operating cycle. These neutron sources typically maintain a fixed position. In the larger reactors, the sources are inserted in selected fuel assemblies and extend within fuel assembly guide thimble tubes designed to receive movable control rod (element) assemblies. They are therefore inserted within fuel assemblies that do not receive a moveable control rod assembly. They are also disposed in assemblies close to the core periphery so as to be positioned close enough to activate the detection and monitoring apparatus outside of the reactor vessel. As the sources remain within an assembly for an entire core cycle, the primary sources, excluding those of Californium, are consumed by neutron induced fission and transmutation before the end of the first fuel cycle, when exposed to the intense neutron flux levels characteristic of power operation. Californium may continue to be active for more than one fuel cycle. As replacement of primary neutron sources with primary neutron sources is costly, the secondary sources, activated under the high neutron flux experienced during its first cycle of operation, are used as replacements.
Typically, a nuclear fuel assembly comprises a bundle of nuclear fuel rods and a support skeleton for those rods. The skeleton comprises a lower nozzle, an upper nozzle and guide thimble tubes which connect the two nozzles and which are intended to receive the rods of movable control rod assemblies for controlling the neutron flux level and thus the operation of the core of the nuclear reactor.
Each movable control rod assembly comprises a bundle of neutron absorbing rods which are retained by a support. This support is generally referred to as a “spider” and is constituted by an upper hub around which fins (also referred to as flukes) are distributed and provided with members for mounting the neutron absorbing rods.
During an operating cycle of the core, the movable control rod assemblies will be displaced in order to introduce to a greater or lesser extent their rods into the corresponding guide thimble tubes and thus to control the reactivity in the core of the nuclear reactor.
In the nuclear reactor core, some nuclear fuel assemblies are not provided with movable control rod assemblies but instead are provided with fixed core rod assemblies that are not subjected to controlled movement during an operating cycle of the core.
This is particularly the case for burnable poison rod assemblies. At least some of the rodlets thereof comprise burnable neutron poison rodlets which will allow the concentration of boron dissolved in the water of the cooling system to be reduced, primarily at the beginning of a cycle.
This is also the case for thimble plug rod assemblies with which some fuel assemblies are provided. The rodlets of these thimble plug rod assemblies occupy the guide thimble tubes of the relevant fuel assemblies in order to limit the hydraulic flow. Typically, at least some of the thimble plug rod assemblies and/or some of the burnable neutron poison rod assemblies are provided with neutron source elements and at least one of the neutron source elements may be a primary neutron source element. Each of the assemblies containing a primary source element, also referred to as a primary neutron source rod assembly, has at least one primary source rodlet assembly that contains a primary radioactive material such as Californium-252 source material, inside a source capsule assembly, which spontaneously emits neutrons and is used for one cycle during the initial core loading and reactor start-up. After the first cycle of operation, throughout the remaining cycles, neutrons are supplied by the secondary neutron source rod assemblies, as mentioned above. The primary neutron source rod assemblies are typically removed after the first cycle, but can remain in the core through the first three cycles and are then retired with the spent fuel assembly that they occupied.
Typically, the primary neutron source rod assembly includes at least one primary neutron source rodlet assembly and fifteen to twenty-three (depending on fuel assembly array type) core component spacer rodlets (also known as thimble plug rodlets) or burnable neutron poison rodlets, connected to a stationary spider assembly or hold down assembly (mounting assembly). Such a mounting assembly is described in U.S. Patent Publication 2010/0111243, published May 6, 2010. The thimble plug rodlets or burnable neutron poison rodlets increase the weight to aid the insertion of the primary neutron source assembly into a fuel assembly. The primary neutron source rodlet assembly is typically fabricated by a subcontract manufacturer while the remainder of the primary neutron source rod assembly (core component assembly) is fabricated and assembled by the nuclear reactor system original equipment manufacturer (OEM). Typically, the primary source rodlet assembly component parts are supplied by the reactor original equipment manufacturer, and the subcontract manufacturer assembles them along with the source capsule assembly according to approved procedures. The core component assembly, which includes the mounting assembly and thimble plug rodlets or burnable neutron poison rodlets, is fabricated by the reactor original equipment manufacturer. The core component assembly and the primary neutron source rodlet assembly are shipped separately to the assembly site location and are then assembled together on site per the appropriate assembly procedures. The site assembly procedures must address the safety requirements associate with a large irradiated component, and are typically written for specific plant sites and source configurations. Alternatively, the primary source rodlet assembly could be manufactured by the reactor original equipment manufacturer if hot cell facilities are available, but even then it's likely the primary neutron source rodlet assembly and the remainder of the primary neutron source rod assembly (core component assembly) would be shipped separately due to the differences in the packaging and shipping requirements.
The primary neutron source rodlet assembly typically comprises a top extension end plug which is adapted to be connected to the stationary spider assembly or hold down assembly, a stainless steel cladding, a bottom end plug and a source capsule assembly held in a selected location with stainless steel or aluminum oxide spacers. Sometimes there is a spring clip to hold things together. There is a stainless steel protective cap used to protect the upper end plug threads during shipping. However, the cap is discarded once the primary neutron source rodlet assembly is assembled into the core component assembly.
The source capsule assembly consists of inner and outer capsules encasing a neutron source material. The source material, Californium Cf252, for example, is used in the form of Californium Oxalate/Palladium composite wire with nearly 1.19 millimeter×1.19 millimeter square cross section that will pass through a 1.91 millimeter diameter hole. The individual wire is then cropped into segments containing approximately 200-500 micrograms of Californium Cf252 depending on the need of source strength at start-up. Sufficient amounts of wire, producing a calculated neutron emission of the required source, are sealed in the inner capsule. Some neutron emission strength requirements will result in a two-wire case and some will be acceptable with a one-wire case.
The source capsule assemblies are made of type 308 stainless steel and sealed hermetically by welding. The capsule assemblies may also be made of zirconium alloy or a similar compatible material. A Californium Oxalate/Palladium wire is captured inside the inner capsule assembly. Sometimes the inner and/or outer capsule is back-filled with helium for better heat transfer. The source capsule assembly is designed to withstand core operational conditions, such as temperature, pressure and irradiation effects. The source capsule is also designed to safely reside in the spent fuel pool for the rest of the plant life.
The primary source rodlet assembly must contain a sufficient curie level of Californium-252 to provide count rates of at least two counts per second at the nearest source range detector for at least six months after the initial start-up of the first reactor cycle. The axial location of the center of the source material should be approximately aligned with the axial location of the center of the source range detectors on the outside of the reactor vessel. FIG. 1 shows the current design flow chart. The primary neutron source rodlet assembly component parts are typically supplied by the reactor original equipment manufacturer and assembled by a subcontract manufacturer along with the source capsule assembly, in accordance with approved procedures. The core component assembly, which includes the thimble plug rodlets and/or burnable neutron poison rodlets and mounting assembly, is fabricated and assembled by the reactor original equipment manufacturer. The core component assemblies and the primary neutron source rodlet assemblies are shipped separately to the site location and then assembled together on site. The site assembly procedures must address the safety requirements associated with a large irradiated component, and are typically written for specific plant and source configurations.
There are a number of disadvantages associated with the current design and manufacturing process. First of all, the primary neutron source rodlet assemblies are shipped in a special shipping container that is about fifteen to twenty feet long and about six feet in diameter. The container is shipped to the assembly site by air, because time is of the essence, and, not all air carriers are equipped and licensed to handle such a large container. In addition, the cost for such a shipment is considerably high, especially for overseas shipments. Secondly, a large enough hot cell is required to accommodate the entire primary neutron source rodlet assembly during the welding of the upper end plug. Also, testing of the upper weld needs to be performed in the same hot cell. However, the hot cell is not required when the lower end plug is welded, because that occurs before the source capsule assembly is encapsulated in the rodlet. Thirdly, once the work on the primary source rodlet assemblies in the hot cell is complete, they are transferred to the large shipping container. During this transfer, there is a significant risk of over exposure to the operators as the primary source rodlet assembly needs to leave the hot cell before it is inserted into the shipping container. Lastly, special equipment is required at the site to handle the large shipping container.
Accordingly, a new neutron source rod assembly design is desired that will facilitate an improved manufacturing process that will overcome the foregoing disadvantages.
Additionally, a new neutron source rod assembly design is desired that will reduce the size and awkwardness of the irradiating components that requires special handling when shipped to the assembly site.
Furthermore, a new manufacturing process for the neutron source rod assemblies is desired that reduces the cost and radiation exposure encountered in handling and shipping the individual assemblies to the assembly site.